The field of the present invention is an automotive vehicle front suspension lower control arm rear bushing. More particularly, the present invention relates to an apparatus and method of utilization of a front suspension lower control arm rear bushing which substantially diminishes rotating mass imbalance sensitivity without significantly increasing vehicle ride harshness.
Some front wheel drive vehicles, especially smaller vehicles, are extremely sensitive to rotating mass imbalance. In some instances, a small 3.5 inch-ounce of imbalance can cause the steering wheel of a very small vehicle to oscillate. Empirical evidence has shown that the mechanism of steering wheel oscillation starts with a rotating mass imbalance of the wheel, tire, rotor and hub imparting a force to the front wheel spindle. The force on the spindle can sometimes be large enough to excite the natural frequency of what is typically referred to as the unsprung mass (wheel, tire, rotor, brake caliper, knuckle, lower control arm, etc.). This so-called unsprung mass is in actuality sprung on four springs. The four springs are the tire sidewall, a strut mount bushing, and the lower control arm front and rear bushings.
As the unsprung mass begins to resonate, it mechanically drives the steering gear back and forth, which causes the steering wheel to oscillate. In some small vehicles the peak steering wheel oscillation can occur in the vicinity of 17 hz. A 17 hz oscillation typically corresponds with a vehicle speed of 72 mph.
Typically, to reduce steering wheel oscillation, it is desirable to move the natural frequency of the unsprung mass beyond the normal operating range of the vehicle. Accordingly, it is desirable to move the natural frequency of the suspension system above 120 mph, which is typically above or at least in the far extreme range of operation of most commercial vehicles in North America. At a speed of 120 mph, it is typical that the natural frequency will move above 30 hz.
Various factors typically control the natural frequency of the suspension system. One factor is the area moment of inertia, which is controlled by the shape and location of the suspension system brake and power train components. If these parameters are defined by other performance characteristics, little change in these parameters is available to reduce the susceptibility of the suspension system to rotating mass imbalance. Another factor that controls the natural frequency is the lever arm that the force acts upon, or as better defined, the distance of the force to the instant center of the unsprung mass. Again, in many instances, these parameters will be predefined by other operational parameters of the vehicle.
One technique to modify the natural frequency of the suspension system would be a drastic reduction in the mass of the suspension system. In most instances, this is not a viable option due to strength requirements of the suspension system.
Another technique to modify the natural frequency of the suspension system is to change the elastic modulus of the softest spring in the system. In a suspension system, the softest spring in the system controls the resultant value of the elastic modulus of the system. In a front suspension system, the lower control arm rear bushing defines the softest spring. Prior to the present invention, attempts were made to provide a higher natural frequency for the suspension system by stiffening the lower control arm rear bushing. However, substantial increases in bushing stiffness typically resulted in a vehicle having an unacceptable harsh ride.
Still another technique to eliminate vibrations in front suspension systems was to dampen the vibrations by utilizing a mass damper or a fluid damper such as a hydro bushing. The utilization of dampers and hydro bushings not only added complexity to the suspension system but also significantly increased costs, which is a critical factor when the front suspension is utilized on a small economy vehicle.
It is desirable to provide a low cost method to reduce vehicle sensitivity to rotating mass imbalance in the front suspension of automotive front wheel drive vehicles.
To make manifest the above-delineated and other manifold desires, the revelation of the present invention is brought forth. In a preferred embodiment, the present invention provides a bushing for a suspension system of an automotive vehicle. The bushing has an inner sleeve made of a hardened material. Annularly spaced from the inner sleeve is an outer sleeve also made from a hardened material. The outer sleeve has an upwardly generally perpendicular extending flange. Juxtaposed between the inner and outer sleeves is a non-fluid filled elastomeric isolator. The isolator is bonded to both sleeves. The isolator has transversely aligned solid sections. The isolator also has first and second generally arcuate axial slots geometrically spaced with one another. The first slot has a major arcuate portion with a generally uniform axial first thickness. The second slot has a major arcuate portion with a non-uniform axial second thickness. The second slot on an axial upper end has a thickness more than 1 millimeter and less than the thickness of the first slot. The second slot thickness will taper axially down to a lower one-third portion wherein the fore and aft walls of the slot will be in contact with each other. The bushing of the present invention is less prone to a natural frequency resonance and has a significantly higher natural frequency from that of prior bushings having generally identical slots. Accordingly, the natural frequency of the bushing is increased and the sensitivity of the suspension system to rotating mass imbalance is greatly diminished.
It is a feature of the present invention to provide a lower control arm rear bushing for a front wheel drive automotive vehicle suspension.
It is a feature of the present invention to provide an automotive vehicle with a front suspension that is less susceptible to vibration caused by rotating mass imbalance.
Other features of the invention will become more apparent to those skilled in the art upon a reading of the following detailed description and upon reference to the drawings.